To date, lighting systems have been supplied in which a plurality of lighting apparatuses and a master for controlling the lighting apparatuses are connected via a communication line (see, for example, Japanese Laid-Open Patent Publication No. 2002-289373, hereinafter, referred to as Patent Literature 1). In the lighting system, ID numbers are uniquely assigned to the lighting apparatuses, respectively, in order to identify the lighting apparatuses, and only when, in each lighting apparatus, an ID number that is included in a communication signal transmitted from the master is identical to the ID number assigned to the same lighting apparatus, the lighting apparatus receives a control signal included in the communication signal, and operates according to the control signal.
In the lighting system, the ID numbers are assigned in the following method. When all the lighting apparatuses and the master have been installed and then powered on, the lighting apparatuses firstly generate respective random numbers and send the respective random numbers to the master, and the master issues provisional ID numbers to the respective lighting apparatuses according to the order of the random numbers, to assign the ID numbers to the respective lighting apparatuses.
Further, another method for allocating addresses with the use of random numbers is also disclosed. In this method, when a random number transmission instruction is transmitted from the master to each of the lighting apparatuses, each of the lighting apparatuses generates a random number, and sets its own timer so as to count to a time obtained by multiplying the generated random number by 10 ms, and transmits, to the master, a pulse signal at a time when the counting has been ended. On the other hand, the master measures, by using its own timer, a time period from a time when the random number transmission instruction is transmitted to each of the lighting apparatuses to a time when the pulse signal is received, and obtains the random number of each of the lighting apparatuses based on the measured value, to allocate addresses based on the obtained random numbers.
In the lighting system disclosed in Patent Literature 1, although the ID numbers can be automatically assigned with ease by using the random numbers, a problem may arise that, since the lighting apparatuses individually generate the random numbers, respectively, if the same random number is generated, by coincidence, among some of the lighting apparatuses, the lighting apparatuses cannot be identified.
Further, in a case where addresses are allocated to the respective lighting apparatuses in the latter method, when the timer of each of the lighting apparatuses and the master is formed as a typical ceramic oscillator, each of the timers has a maximum deviation of 0.5%, and thus, an error may occur between a random number obtained by measuring the time in the master, and a random number generated by each of the lighting apparatuses.